Centaurea solstitialis
Centaurea sulphurea
Barnaby star-thistle, centauré du solstice, golden starthistle, St. Barnaby's thistle, yellow cockspur, yellow knapweed, yellow star-thistle
Sicilian star-thistle, sulphur knapweed, sulphur-color Sicilian thistle
simple or often branched from base, forming rounded bushy plants, gray-tomentose.
simple to openly branched, branches ascending, villous to hispid with septate hairs and loosely tomentose.
gray-tomentose and scabrous to short-bristly;
basal and proximal cauline petiolate or tapered to base, usually absent at anthesis, blades 5–15 cm, margins pinnately lobed or dissected;
cauline long-decurrent, blades linear to oblong, 1–10 cm, entire.
± villous to hispid with septate hairs, minutely resin-gland- dotted;
basal winged-petiolate, blades oblong to oblanceolate, 10–15 cm, margins pinnately lobed, lobes acute, finely dentate;
cauline sessile, long-decurrent with narrow wings, linear-oblong to oblanceolate, 1–6 cm, entire or distally serrate with short, spine-tipped teeth.
ovoid, 13–17 mm, loosely cobwebby-tomentose or becoming glabrous.
ovoid, 12–30 mm, distally constricted.
many;
corollas yellow, all ± equal, 13–20 mm;
sterile florets slender, inconspicuous.
many;
corollas yellow, all ± equal, 25–35 mm;
corollas of sterile florets slender, inconspicuous.
appendages scarious, obtuse or abruptly spine tipped.
appendages acute or spine-tipped.
disciform, borne singly or in open leafy arrays, long-pedunculate.
disciform, borne singly or in open, few-headed corymbiform arrays, long-pedunculate.
dimorphic, 2–3 mm, glabrous, outer dark brown, without pappi, inner white or light brown, mottled;
pappi of many white, unequal bristles 2–4 mm, fine.
dark brown, 5–8 mm, glabrous;
pappi of many, brown to blackish, unequal bristles 6–7 mm.
phyllaries: bodies pale green, ovate, appendages stramineous to brown, each with palmately radiating cluster of spines, and stout central spine 10–25 mm.
phyllaries: bodies greenish or stramineous, ovate to elliptic, glabrous, appendages spreading to reflexed, brown to blackish purple, each with palmately radiating cluster of spines, central spine stout, 1–2.5 cm, base dark brown to black, distally stramineous.
= 16.
= 24.
Centaurea solstitialis
Centaurea sulphurea
Centaurea solstitialis is a serious weed pest, especially in the western United States, where it has invaded millions of acres of rangelands, and it is listed as a noxious weed in eleven western states and two Canadian provinces. It is a strong competitor in infested areas, often forming dense colonies. It is very difficult to control or eradicate once it becomes established. In addition, yellow star-thistle is poisonous to horses; when ingested over a prolonged period it causes a neurological disorder called equine nigropallidal encephalomalacia, or “chewing disease.” Although its bitter taste and spiny heads usually deter grazing animals, horses sometimes will seek it out. Yellow star-thistle tends to spread in rangelands when more palatable plants are consumed.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Centaurea sulphurea is considered to be a noxious weed by the state of California.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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OR, 
WA - Local Web sites: CalFlora, CalPhotos,
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MI Flora, 
MN Wildflowers, 
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PNW Herbaria, Turner Photog. 
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